The obvious problem with Credits is the 30% cut Facebook will take from all transactions. This led to a public showdown between Facebook and its biggest partner, FarmVille creator Zynga, though the two companies have since reached an accommodation.

But Eric points out two other worries that some developers have about Credits becoming universal:

Breakage: Not all of the virtual currency the users purchase ever gets spent. Often, users spend money, then lose interest and walk away. This is called 'breakage'. Many game companies treat this as money already spent, but Facebook, following California law, treats it as "abandoned property" which defaults to the state after three years.

Optimization: Social games generally don't have a single, fixed price for their currencies. Instead, they offer discounts for buying larger amounts of currency at once, and hold periodic limited time sales. If virtual goods were priced in a currency controlled by Facebook, they wouldn't be able to do any of that.

These concerns strike us as overblown. To understand why, it's important to distinguish between two different ways games use Credits. Some developers have started to use Credits as the primary currency for purchasing individual virtual goods within games. More commonly, however, developers have their own in-game currency, and make Credits one of the ways users can buy that currency. This adds another step to the purchasing process for users (real money --> Credits --> game currency --> virtual goods), but leaves developers free to optimize the sale of their currency, and lets them hang on to the proceeds of abandoned currency.

Facebook has expressed its interest in becoming the universal payment system on the transaction level; that is, it wants all real money entering the Facebook ecosystem to flow through Credits (though so far, it appears it wants to beat other payment options, rather than banning them). But there is no reason to believe Facebook would ever make developers use Credits inside their games.

There are still downsides to this; it would add an extra step, at least for first time purchasers. But there is a huge potential benefit for developers, too: if Facebook can make Credits take off, so that the average user has some lying around in his account at any given time, then Facebook gamers will be a click away from paying up from the time they start playing. That could be worth a lot more than the 30% developers are giving up.